This invention relates generally to optical elements and, in particular, to multiplexed gratings and combination grating/prism devices for use DWDM optical telecommunications networks.
Optical telecommunications over optical fibers is now the preferred mode of high-bandwidth data transmission in comparison to copper wire, particularly over long distances. Such systems use lasers modulated in amplitude by the data to be transmitted. The signals are coupled into an optical fiber for detection and demodulation at the other end of the link.
The existing infrastructure of long-haul optical fiber is rapidly becoming taxed to its bandwidth capacity. Laying more fiber to carry additional bandwidth is an extremely expensive proposition. Dense wavelength-division multiplexing (DWDM) has emerged as a more cost-effective solution. The idea is to force existing fibers to carry more bandwidth by combining signals from multiple lasers operating at different wavelengths onto a single fiber. Optical multiplexers take optical signals at different wavelengths propagating on different fibers and combine them onto a single fiber. Optical amplifiers based on pumped erbium-doped fibers amplify the entire set of multiplexed optical signals on a fiber for transmission over long distances.
A critical element of a DWDM optical network is the Optical Spectrum Analyzer (OSA), which monitors the spectral content of optical signals propagating on a fiber. The OSA measures the presence, wavelength position, amplitude, and signal-to-noise ratio of all of the optical signals being carried on a fiber. The input to the OSA is normally a small fraction of the light within the fiber being monitored, picked off by a fiber tap coupler. The OSA measurements are essential to maintaining the integrity of the optical network by monitoring its essential functions. OSAs are also used in feedback loops to dynamically adjust amplifier gain profiles, so that all of the optical signals within the fiber can be maintained at the same amplitudes.
Two different wavelength bands for DWDM are currently in use: the xe2x80x9cC-bandxe2x80x9d (central or conventionalxe2x80x94approximately 1530 to 1565 nm) and the xe2x80x9cL-bandxe2x80x9d (long wavelengthxe2x80x94approximately 1575 to 1610 nm). These bands are largely defined by fiber and amplifier technologies. Up to approximately 100 different wavelength channels, each separated by 50 GHz (approximately 0.4 nm) may be multiplexed onto a single fiber in either band.
It is possible for C-band and L-band equipment to co-exist at a common network node. In these installations, a dual-band OSA can provide required functionality at significantly lower cost than separate single-band OSAs. C-band and L-band signals usually exist on separate fibers at these nodes, although it is physically possible for them to coexist in a single fiber.
OSAs for DWDM network monitoring are currently available using two fundamentally different technologies: dispersive gratings, and scanning Fabry-Perot cavities. Scanning Fabry-Perot devices are available from Queensgate Instruments, Ditech Communications Corp., and Micron Optics, Inc. to name a few. These devices are basically tunable filters that scan the wavelength range of interest. They are available in versions that cover either a single band or both the L-band and C-band, depending on the free spectral range of the cavity design. A significant disadvantage of scanning Fabry-Perot OSAs is the fact that they have moving parts that must be actuated to generate the spectral scan. This has undesirable implications on long-term reliability, environmental durability, and calibration stability. Also, the spectrum of interest is acquired sequentially rather than simultaneously.
Dispersive OSAs use diffraction gratings to spatially separate the spectral components of light within the DWDM network. Grating-based OSAs come in at least three fundamentally dissimilar configurations. One is based on a scanned grating, wherein the grating angle is changed to bring a single wavelength to image onto a single detector. Scanned grating devices have similar inherent disadvantages for long-term, continuous use installations due to their moving parts. An advantage is that they can be scanned over wide wavelength ranges at fairly high resolution, including the full C and L bands in a single instrument. Due to reliability issues, scanned grating OSAs have been largely relegated along with scanning Fabry-Perot filters to laboratory test instruments.
The more robust forms of grating-based OSAs for network monitoring use stationary gratings in an xe2x80x9cimaging spectrographxe2x80x9d configuration. That is, the grating spreads the spectrum angularly, and then a lens images the entire spectrum of interest onto an array of detectors, rather than a single detector. In this configuration, data for the entire spectrum is gathered simultaneously without any moving parts.
Two varieties of grating-based OSAs are available. One, shown schematically in FIG. 1, uses a holographic grating/prism of the type described in U.S. patent application Ser. No. 09/560,595 to provide extremely high angular dispersion in a very compact configuration with two simple lenses and a linear InGaAs detector array available from Sensors Unlimited and others. This system is characterized by a fold of the optical path through nominally 180xc2x0.
A similar device, made by JDS for L-band monitoring, uses a similar holographic grating/prism, but with a fold of the optical path through nominally 160xc2x0, yielding slightly lower dispersion. The fold angles and wavelength ranges are quite flexible in the design of the grating-based OSA. Either band may be accommodated in either geometry, and other geometries with proper adjustment of other system parameters such as lens focal lengths or detector size.
FIG. 3A shows a prior-art, single-band grating construction. The grating may be constructed as appropriate for use in either the C- or L-fold configuration, or any other configuration useful to DWDM monitoring or routing, with or without prisms. A further OSA configuration, made by Lucent Technology, uses a blazed fiber Bragg grating to angularly disperse the light directly out of the fiber for imaging onto a similar detector array. OSAs based on these varieties are available as either C-band or L-band devices, but not both simultaneously.
The main barriers to providing a dual-band version of the stationary grating OSA are: 1) the lack of moving parts to direct multiple spectral regions onto a common detector or array; 2) the resolution of the linear detector array, currently available in up to about 512 pixels; 3) the efficiency that a stationary grating can maintain over a wide range of wavelengths and angles; and 4) the desire to keep the system compact.
The detector resolution is probably the most significant limiting factor, because in either L-band or C-band, as many as 80 to 100 signal channels need to be monitored and well-resolved by the array. In order to accurately interpolate the center wavelength of each signal peak, more than one or two samples of each peak is required. If one tried to measure the positions of, say, 160 to 200 peaks in both bands with only, say, 256 detectors in a linear array, the signals could not even be separated due to the Nyquist limit (2 samples per signal), let alone an accurate estimation of wavelength position.
This invention resides in multiplexed grating and grating/prism devices, particularly suited to DWDM optical telecommunications networks. As a primary application, a device according to the invention may serve as a key element of an optical spectrum analyzer (OSA). Such a device may also be used as an element of a fiber multiplexer or demultiplexer, by replacing the described detector array with a fiber or waveguide array.
Slightly different versions of the current invention can be used to address both single- and dual-band configurations. In the latter case, a modified detector array would be used relative to a single-band OSA, and the grating orientations would be slightly different. In the former case, the only difference between the single-band OSA and the dual-band OSA is the holographic layers of the grating or grating/prism, and the addition of a fiber-optic switch. As a dual-band OSA covering the C- and L-bands, a device according to the invention may be used to replace two separate OSAs like those currently in production, at only a modest increase in cost relative to a single-band OSA.